Radio Link Control (RLC) protocol layer is a sub-layer of Layer 2 (L2) in radio interface protocol stack of Long Term Evolution (LTE), located between a Media Access Control (MAC) layer and a Packet Data Convergence Protocol (PDCP) layer. The RCL protocol layer has functions including link control, segmentation, reorganization, cascading, user data transmission, error correction, protocol error detection, restoration and so on, and provides segmentation and retransmission services for user and control data.
Each RLC protocol entity is configured by the Radio Resource Control (RRC) layer, and includes three data transmission modes: Transparent Mode (TM), Unacknowledged Mode (UM) and Acknowledged Mode (AM).
For an Automatic Repeat Request (ARQ) in the acknowledged mode, first, a status report is sent to a sending terminal by a receiving terminal; and then the sending terminal determines which Protocol Data Unit (PDU) messages have been received by the receiving terminal and which PDU or PDU fragments have not yet been received according to ACK_SN and NACK_SN in the status report, and retransmits the PDU messages that have not been received to the receiving terminal, thereby guaranteeing the reliability of data transmission.
According to 36.322 protocol, there are two ways for triggering the status report: the RLC sending terminal triggers the status report by means of polling (a polling timer is time out); the RLC receiving terminal detects a failure of the PDU reception (a rearrangement timer is time out). In the 36.322 protocol, the packaging process of status report under the acknowledged mode is described as follows: for PDUs that have not been received completely within the rearrangement detection interval of the receiving window, the status report is packaged according to the serial numbers of the PDUs (that is, the serial numbers of the messages that have not been received completely) ascending from smaller to larger in turn, until the size of the status report meets the size of the available bandwidth indicated by the lower layer.
FIG. 1 and FIG. 2 respectively show schematic diagrams when the data transmission window stays in moving status and halt status in related technologies, wherein the window sizes of both the sending window and the receiving window are Window_Size=512, window element being shadow indicates that a whole PDU has been received, half of the window being shadow indicates that only a part of the PDU has been received, still one or more PDU fragments have not been received.
FIG. 1 shows the schematic diagram when the data transmission window stays in moving status in related technologies. Specifically, a minimum serial number of the PDU that has not been received by the receiving window is i>0, that is, VR(R)=i>0, while VR(H)=512, so VR(H)−VR(R)>512=Window_Size, indicating that the receiving window is not full and is in moving status. At this time, the sending window is also in moving status, therefore VT(S) can move backwards to send new message data.
However, during transmission of air interface data, a case in which the window stays in halt status is also possible. FIG. 2 shows the schematic diagram when the data transmission window stays in halt status in related technologies. Specifically, the minimum serial number of the PDU that has not been received by the receiving window is 0, that is, VR(R)=0, while VR(H)=512, so VR(H)−VR(R)=512=Window_Size, indicating that the receiving window is full and stays in halt status. When the receiving window is in halt status, the sending window is also undoubtedly in halt status (as Window_Size=VT(S)−VT(A), at this time, VT(S)≧VR(H) and VT(A)≦VR(R)), and VT(S) is unable to move backward to send new message data. The halt status ends only when the PDU with the serial number of 0 is retransmitted by the sending terminal and acknowledged of its receiving by the receiving terminal. Therefore, when the window stays in halt status, the data transmission is interrupted and the transmission speed of air interface data is seriously affected.
FIG. 3 shows a schematic diagram of the status report in related technologies. As shown in FIG. 3, each line of 8 bits in the status report represents 1 byte, and each status report at least includes one ACK_SN, several NACKs and several E1s and E2s. ACK_SN (10 bits) represents the final serial number position of the status report detection interval, that is, what the status report reports to the sending terminal is the reception situation of messages with the serial numbers before the ACK_SN. NACK is composed of NACK_SN and SO domain (optional), wherein NACK_SN (10 bits) indicates that the PDU or PDU fragment with the message serial number being SN has not been received. When it is the PDU fragment that has not been received, the NACK further includes an SO domain including an SOstart (15 bits) and an SOend (15 bits) for indicating the start position and end position of the unreceived PDU fragment in the PDU. The E1 is used for identifying whether there is a further NACK following the ACK_SN/NACK, and E2 is used for identifying whether there is an SO domain following the NACK_SN.
Scheduling bandwidth, also called available bandwidth, refers to that available scheduling bandwidth allocated for RLC by the MAC according to the size of message data to be sent when there is message data to be transmitted in the RLC buffer and a Buffer Status Report (BSR) is sent to the MAC. The available scheduling bandwidth is generally unequal to the size of the message data to be sent, and the packaging principle of RLC is to send message data as much as possible according to the size of the available bandwidth. When RLC retransmits a PDU, if the scheduling bandwidth is greater than the size of the data to be sent, MAC layer needs to add a Padding after the MAC PDU to adapt to the scheduling bandwidth; if the scheduling bandwidth is smaller than the size of the message data to be sent, the PDU to be retransmitted needs to be divided into fragments. Under good air interface situation, generally, the status report can be scheduled sufficiently, that is, the NACKs corresponding to the rearrangement detection interval of the receiving terminal can all be sent to the sending terminal; while under poor air interface situation, the status report is sent as much as possible according to the size of the bandwidth.
Under certain conditions, RLC entity receiving terminal has retransmission requirements for certain PDU fragments, if the available bandwidth is insufficient at this time so that the NACK with the message serial number being VR(R) is not included in the status report sent to the sending terminal by the receiving terminal, only when a new turn of status report is triggered when the arrangement timer or polling timer is time out again, it is possible to construct the NACK with the message serial number being VR(R) in a status report of a new round to request the sending terminal to retransmit PDU fragments corresponding to VR(R).
A method for message retransmission has been disclosed in related technologies, wherein when the receiving terminal constructs a status report, an NACK truncated by the available bandwidth is abandoned without sending, and only the NACK before the truncated NACK is constructed into the status report to be sent. As shown in FIG. 3, the available bandwidth of RLC is only 13 bytes far less than the length of the status report corresponding to the arrangement detection interval, the SOend domain of the third NACK is just truncated by the available bandwidth and unable to be sent completely. At this time in the method for message retransmission, only a status report including the previous two NACKs is sent and the third NACK truncated by the available bandwidth is abandoned, that is, only a status report of 9 bytes is sent.
The inventor found that in the method for message retransmission in related technologies, the NACK truncated by the available bandwidth is abandoned no matter under which status of the data transmission window, resulting in that under a case in which the window is full to halt, the halt status can only be released after at least one arrangement timer cycle or polling timer cycle, thereby resulting in air interface data transmission delay and low transmission speed.